1. Field of the Invention
The present invention relates to voltage control technology and more particularly, to a soft-start voltage regulating circuit composed of active components and passive components. The invention relates also to a generator voltage regulating system using the soft-start voltage regulating circuit.
2. Description of the Related Art
During running of a car, the desired electric power is supplied by a generator. If the generator is not yet started, the stator and excitation coil of the generator have no electric current passing therethrough and do not generate power supply. It is necessary to provide current to the excitation coil to cause the stator to work, enabling the generator to generate power supply. Generator excitation current source and control method can be achieved through a pre-excitation circuit. Some known pre-excitation circuits include a trio diode. Some other known pre-excitation circuits do not include a trio diode. FIG. 1 illustrates a trio diode-included pre-excitation circuit according to the prior art. According to this design, before running of the generator, excitation current Ig goes from node N to conduct a start switch 71 and an indicator light 72. This means that excitation current Ig comes from the start switch 71. After the generator has been started to establish voltage, excitation current Ig goes through a trio diode 73 to provide the necessary current.
FIGS. 2 and 3 illustrate a trio diode-excluded pre-excitation circuit according to the prior art. The circuits 70a and 70b are made by integrated circuit (IC) in the prior art, and the duty cycle is fixed at a specific value during IC manufacturing, for example, 12%, 18% or 30%. The IC design and manufacturing lacks of adjustment flexibility of duty cycle, besides, the cost for developing specific duty cycle of the integrated circuit is high.
Therefore, it is desirable to provide a soft-start circuit design, which provides adjustment flexibility on duty cycle and reduces the circuit manufacturing cost.